The present invention relates to an electromagnetic actuator which is capable of a highly accurate control.
Electromagnetic linear actuators having, a form in which a step motor is one-dimensionally expanded are known. Such electromagnetic linear actuators are disclosed in, for example, Japanese Utility Model Publication No. 1290/1973. This electromagnetic actuator includes a rod made of a magnetic substance and having teeth at a fixed pitch or a rod on which permanent magnets are disposed at a fixed pitch and a plurality of electromagnets whose surface is toothed in the same manner as that of the rod, with the electromagnets being disposed on the rod. Pulse current is systematically supplied to the individual electromagnets, by which a magnetic field is shifted. A driving force is generated because of the positional offsets between the teeth on the surface of the rod and the teeth on the surfaces of the electromagnets.
Japanese Utility Model Laid-Open No. 116887/1981 discloses an electromagnetic linear actuator which includes a cylindrical coil and a rod made of a magnetic or ferromagnetic substance and disposed on a central axis of the coil. In this electromagnetic linear actuator, a driving force is generated because of an offset of dynamic balance within the magnetic field which is generated by the shape and position of the rod.
However, in the electromagnetic linear actuator of the type disclosed in Japanese Utility Model Publication No. 1290/1973, the magnetic flux density obtained by the energization of the electromagnets is not symmetrical with respect to the axis of the rod, and a balance of the force between the rod and the electromagnets thus varies greatly. This causes a force having a large magnitude to act on the rod retaining mechanism, and makes enhancement of accuracy and response of the actuator control difficult.
In the electromagnetic linear actuator disclosed in Japanese Utility Model Laid-Open No. 116887/1981, a driving force differs depending on the position of the rod, and stroke is small. Furthermore, although the rod can be moved digitally by conducting pulse control on the rod using a stator which temporarily restricts the movement of the rod, it is difficult to quantitatively set the amount of movement of the rod when the movement of the rod is not restricted by the stator. This makes enhancement of the actuator control accuracy impossible.